Audio signal reproduction has evolved beyond simple stereo, or dual-channel, configurations or system. For example, surround sound systems, such as 5.1 surround sound, are commonly used in in-home and commercial installations. Such systems employ loudspeakers at various locations relative to an expected listener, and are configured to provide a more immersive experience for the listener than is available from a conventional stereo configuration.
Some audio signal reproduction systems are configured to deliver three dimensional audio, or 3D audio. In 3D audio, sounds are produced by stereo speakers, surround-sound speakers, speaker-arrays, or headphones or earphones, and can involve or include virtual placement of a sound source in a real or theoretical three-dimensional space auditorily perceived by the listener. For example, virtualized sounds can be provided above, below, or even behind a listener who hears 3D audio-processed sounds.
Conventional stereo audio reproduction via headphones tends to provide sounds that are perceived as originating or emanating from inside a listener's head. In an example, audio signals delivered by headphones, including using a conventional stereo pair of loudspeaker drivers, can be specially processed to achieve 3D audio effects, such as to provide a listener with a perceived spatial sound environment. A 3D audio headphone system can be used for virtual reality applications, such as to provide a listener with a perception of a sound source at a particular position in a local or virtual environment where no real sound source exists. In an example, a 3D audio headphone system can be used for augmented reality applications, such as to provide a listener with a perception of a sound source at a position where no real sound source exists, and yet in a manner that the listener remains at least partially aware of one or more real sounds in the local environment.